Scientific and patent literature contain numerous references to the sorbing action of natural and synthetic molecular sieve zeolites. For the separation of components of bulk gas mixture and the purification of gas streams containing a minor amount of accompanying contaminant, such molecular sieves are most generally employed in pelletized form of zeolite crystals containing an alkali metal cation, particularly in sodium form, or, in some instances, at least part of the sodium has been replaced by a divalent or trivalent cation.
A large number of diverse systems have been described in prior art patents for separation of components of a gas mixture by selective adsorption, employing molecular sieve zeolites. Among these are systems particularly designed or asserted to be useful in the recovery of an enriched oxygen product stream from air, by adiabatic operation wherein selective adsorption, generally of the nitrogen component, is effected at higher pressure and subsequently desorbed at reduced pressure. Typical of such systems are those described in U.S. Pat. Nos. 2,944,627; 3,564,816; 3,636,679; 3,738,087; 3,796,022; 3,923,477 and 4,013,429.
Although certain naturally occurring zeolites have been mentioned as useful in various gas separation processes, the predominant choice for air fractionation has been commercially available synthetic aluminosilicates, such as 5A (a sodium aluminosilicate partially base exchanged with calcium) or 13X. (See, for example, U.S. Pat. Nos. 3,164,454; 3,280,536 and 3,796,022). In other patents directed to air fractionation the adsorbent of choice is pelleted sodium mordenite (U.S. Pat. Nos. 3,957,463; 4,013,429 and 4,264,340).
For use as catalysts in hydrocarbon conversion processes natural and synthetic alkali metal zeolites are employed in base-exchanged form, wherein a greater or less part of the alkali metal cation is substituted by hydrogen, rare earth metal or other cation (U.S. Pat. Nos. 2,882,244 and 3,436,357). For the production of hydrogen zeolites the preferred technique is to substitute at least the major part of the alkali metal cation therein by ammonium exchange, followed by calcination to drive off NH.sub.3. Direct substitution of alkali metal by hydrogen is also known, but less preferred. For example, in U.S. Pat. No. 3,190,939, hydrogen mordenite is advocated for use as catalyst in paraffin isomerization. Conversion of the original sodium mordenite to the hydrogen form is stated to be achieved by direct replacement of sodium ions with hydrogen ions or by replacement of sodium ions with ammonium ions followed by calcination. At least about 95% of the alkali metal is thus replaced by hydrogen.
The specific preparation of pelleted synthetic sodium mordenite is described in U.K. Pat. No. 979,398. To obtain hydrogen mordenite the sodium mordenite is leached with strong mineral acid at a concentration higher than 1N.
Numerous other patents describe base exchange of the major part of the sodium by hydrogen applied to natural and synthetic zeolites, such as 13X and 13Y for use as hydrocarbon conversion catalysts. Hydrogen zeolites have not been advocated for use as selective adsorbents in gas fractionation processes. For such gas fractionation processes, as indicated above, the adsorbent typically employed is the pelleted sodium zeolite (such as 13X or sodium mordenite) or one in which the sodium has been replaced in part by an alkaline earth metal cation, as in zeolite 5A.
For practical commercial operation on a large scale it is necessary that the zeolitic materials, which are normally produced as fine powders, be formed into suitable shapes for loading into an adsorber column, Typically, the material is formed into cylindrical shape of about one-eighth inch in diameter and about 1/8 inch length or beads of about 1/16" to 1/8" in diameter, by methods well known in the art. Thus, the powdered zeolitic material is mixed with water or other liquid to form a paste, with or without inclusion of a binder, and the paste extruded under pressure through dies having the desired diameter and cut or broken to pellets of desired length. The extruded material is then dried and calcined to develop structural strength.
Commercial sodium zeolite pellets of different batches from the same supplier as well as materials from different suppliers, have been found to have important differences in performance quality from the standpoint of adsorption dynamics in columns particularly in nitrogen adsorption from oxygen, although the equilibrium adsorptive properties for nitrogen and oxygen of these adsorbents were comparable. It was conjectured that the poor kinetics displayed by certain batches of the commercial pellets might be attributed to variations in the pelleting operation.
Extensive investigation was carried out in attempt to determine the reasons for the exhibited differences in the quality of different batches of the supplied commercial adsorbent. Scanning electron micrographs of the commercial zeolite pellets showing poorer performance quality revealed that the surface of the pellet was composed of far less crystalline material than the interior. It was also noted that the surface skin appeared to be somewhat impervious. From these and other observations made it was believed that the kinetics of adsorption on commercial zeolite pellets may be adversely affected due to the presence of a surface resistance to mass transfer. In addition to such surface resistance, undue blockage of macropore structure in the zeolite pellets conceivably could also slow down access of the adsorbing molecules to the zeolite crystals. Such blockage, it was postulated, could be caused by the presence of unreacted aluminosilicate in the pellet. Such unreacted material might be caused to fuse during regeneration of the adsorbent in use, or as a result of thermal treatment during production of the pelleted structure, and thus tend to block some of the macropore structure. A research program was therefore initiated further to study the poorer behavior of certain zeolites used in adsorptive gas separation and to find ways to improve the adsorption kinetics of pelleted sodium zeolites.
An "in-house" quality standard for pelleted sodium mordenite adsorbent had previously been established on the basis of consistent equilibrium gas sorption and dynamic sorption characteristics exhibited over a period of several years by different batches of pelleted sodium mordenite obtained from the same commercial source.